Method of wafer band-edge measurement using transmission spectroscopy and a process for controlling the temperature uniformity of a wafer

ABSTRACT

A method and system for using transmission spectroscopy to measure a temperature of a substrate ( 135 ). By passing light through a substrate, the temperature of the substrate can be determined using the band-edge characteristics of the wafer. This in-situ method and system can be used as a feedback control in combination with a variable temperature substrate holder ( 182 ) to more accurately control the processing conditions of the substrate. By utilizing a multiplicity of measurement sites the variation of the temperature across the substrate ( 135 ) can also be measured.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority co-pending applications entitled“A Method of Substrate Band-Edge Measurement Using TransmissionSpectroscopy and a Process for Tuning Temperature Uniformity,” U.S.Provisional Ser. No. 60/174,593 filed Jan. 5, 2000; and “Multi-ZoneResistance Heater,” U.S. Provisional Ser. No. 60/156,595 filed Sep. 29,1999. All of those applications are herein incorporated by reference intheir entirety.

FIELD OF THE INVENTION

The present invention is directed to an in-situ method of measuring thetemperature of a substrate with a temperature dependent band-gap usingband-edge thermometry (BET), and, more specifically, using transmissionspectroscopy (TS).

DISCUSSION OF THE BACKGROUND

The accurate measurement of semiconductor substrate temperatures duringprocessing is highly desirable for semiconductor substrate processing.In particular, most processes are temperature sensitive and therefore,accurate temperature measurement is a pre-requisite to the control ofoptimal conditions for etch and/or deposition chemistry. Moreover, aspatial variation of temperature across a semiconductor substrate canlead to non-uniform processing when either etching or depositingmaterial.

There are three geometric modes or configurations of band-edgethermometry (BET): (1) transmission spectroscopy (TS; see FIG. 1C), (2)specular reflection spectroscopy (SRS; see FIG. 1D), and (3) diffusereflectance spectroscopy (DRS; see FIGS. 1A and 1B). The geometry ofeach mode is presented in FIGS. 1A-1D.

In the DRS mode, the light source and detector are on the same side ofthe substrate with the detector placed in a non-specular position (seeJohnson et al., U.S. Pat. Nos. 5,568,978 and 5,388,909 (hereinafter “the'978 patent” and “the '909 patent,” respectively)). A non-speculardetector only sees the light that is transmitted through the wafer andthat is diffusely back scattered into the solid angle of the detector.In the DRS method, the double-pass transmission of light through thesubstrate is measured as a function of wavelength or, equivalently,photon energy. As the wavelength increases, the photon energy decreases,and the onset of substrate transparency occurs as the photon energybecomes less than the band-gap energy.

In the SRS mode, the light source and detector are also on the same sideof the substrate. The detector is placed in a specular position where itdetects light that is specularly reflected from both surfaces of thewafer (see Cabib & Adel, U.S. Pat. No. 5,322,361 (hereinafter “the '361patent)). The light that is reflected into the detector withouttraveling through the wafer contains no temperature information andconsequently adds only a relatively constant background signal. Thelight component that is reflected from the opposite internal surface ofthe substrate travels back through the wafer and onto the detector. Thatreflected component, which passes twice though the wafer, contains theuseful temperature information.

In the TS mode, the onset of substrate transparency (or, equivalently,the band-gap energy) is determined by the transmission of light throughthe substrate as described in Kirillov & Powell (U.S. Pat. No. 5,118,200(hereinafter “the '200 patent”)). In this geometry, the light source andthe detection system are on opposite sides of the wafer. One difficultywith this approach is that it requires optical access to the chamber atopposite sides of the substrate. However, in comparison to the SRS mode,the TS mode results in an increase in the light intensity received bythe optical detector.

No matter what mode is used, a temperature signature must be extractedfrom the spectra. In general, three algorithms have commonly been usedto extract substrate temperature from band-edge spectra: (1) thespectral position of the maximum of the first derivative or,equivalently, the inflection point, (2) a direct comparison of thespectrum to a predetermined spectral database, and (3) the position ofthe spectrum knee (i.e., the location of the maximum of the secondderivative). The first method has been discussed in the '200 patent.That method determined the substrate temperature as a function of theposition of the inflection point of the spectrum in a previouscalibration run where the temperature of each spectrum is known. Theadvantages of that method are that it is simple, fast and independent ofthe absolute intensity of measurement. The disadvantage is that it isvery sensitive to interference effects that may occur at either surfaceof the processed silicon (Si) wafer.

In the second approach, the '361 patent compares a given spectrum to atemperature-dependent database composed of spectra taken at knowntemperatures. One advantage is that it is reported to work well for Siwafers. A disadvantage is that it is sensitive to interference effectsand requires an absolute reflectivity measurement. Accordingly, eachwafer may require a separate normalization spectrum.

Lastly, the '978 and '909 patents disclose a DRS mode BET, using theposition of the spectrum knee as a signature. Its advantage is that itis the closest distinct point to the onset of transparency of asubstrate, and is therefore less sensitive to interference effects. Ashortcoming of this approach is that it requires sophisticated fittingalgorithms that may be too slow for some current applications.

In general, a BET system includes three main units, i.e., a lightsource, a dispersion device and a photo-detector. Currently, there areseveral commercially available systems; however, none of these systemsis fully capable of the following criteria:

1) Non-contact thermometry from the bare backside of Si wafers duringfront side processing.

2) Use of optical methods and quartz rods to couple light in and out ofthe process chamber.

3) Two-dimensional snapshot of wafer temperature.

4) Simultaneous samples of several points (approximately 10) on large Siwafers with a response time of 100 msec or less.

5) Temperature range of 20 to 300° C.

6) Accuracy of temperature measurement to within 2 to 5° C.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a non-intrusivemethod of measuring (1) substrate temperature and (2) spatial variationof the substrate temperature. This measuring process can, in turn, beemployed to (1) tune the thermal response of a chamber to a process and(2) concurrently modify temperature characteristics of the chamber inresponse to temperature measurements performed in-situ throughout thatprocess.

Since the band-gap of most semiconductor materials decreases withtemperature (linearly above the Debye temperature), the onset oftransparency of semiconductor materials gives a precise reproduciblemeasure of substrate temperature. This makes band-edge thermometry (BET)an ideal method for in-situ non-contact measurements of substratetemperature during semiconductor processing. This method is particularlyuseful for low temperature applications where pyrometry is not effectiveand in applications where the process has a detrimental effect onin-situ temperature sensors (e.g., thermocouples) or, conversely, wherein-situ temperature sensors have a detrimental effect on the process.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will become readily apparent to those skilled in theart with reference to the following detailed description, particularlywhen considered in conjunction with the accompanying drawings, in which:

FIG. 1A is a schematic illustration of a first configuration of anapparatus using diffuse reflectance spectroscopy;

FIG. 1B is a schematic illustration of a second configuration of anapparatus using diffuse reflectance spectroscopy;

FIG. 1C is a schematic illustration of a first configuration of anapparatus using transmission spectroscopy;

FIG. 1D is a schematic illustration of a first configuration of anapparatus using specular reflection spectroscopy;

FIG. 2 is an illustration of a prior art inductively coupled plasma(hereinafter ICP) source;

FIG. 3 is an illustration of a prior art electrostatically shieldedradio frequency (hereinafter ESRF) plasma source;

FIG. 4 is a schematic illustration of a first embodiment of a wafertemperature measurement system that uses feedback to control thetemperature of a substrate;

FIG. 5 is a schematic illustration of a second embodiment of a wafertemperature measurement system that uses feedback to control thetemperature of a substrate;

FIG. 6 is a schematic illustration of a third embodiment of a wafertemperature measurement system that uses feedback to control thetemperature of a substrate;

FIG. 7 is a schematic illustration of a fourth embodiment of a wafertemperature measurement system that uses feedback to control thetemperature of a substrate;

FIG. 8 is a graph showing the normalized transmission of IR radiationthrough a Si substrate as a function of wavelength with temperature as aparameter;

FIG. 9 is a schematic illustration of a wafer including pluralmeasurement sites; and

FIG. 10 is a schematic illustration of a computer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views, FIG. 1Cis a schematic illustration of a first configuration using transmissionspectroscopy. The following sections describe: (1) the fundamentalprinciples behind the use of transmission spectroscopy (TS); (2)embodiments used for measuring temperature, including a description ofthe light source(s), a description of the optics, and a description ofthe detection system; (3) a method for extracting temperatureinformation; (4) the measurement speed; (5) the spectral resolution ofthe measurement, and (6) the tuning of the thermal characteristics of asubstrate.

Fundamental Principles

The basic theory described herein is based on simulations of the Siband-edge when the absorption cross-section is assumed to be constantover the operating temperature and spectral ranges. Furthermore, theabsorption coefficient near the band-edge is assumed to be proportionalto the joint density of states of an indirect band-gap material withparabolic bands. Finally, the simulated band-edge spectra for Si arebased on the TS measurement configuration.

Assuming that absorption is proportional to the optical joint density ofstates and that the energy bands are parabolic, then the absorptioncoefficient is quadratic in energy (for energies above the band-gap (inindirect band-gap materials)). Under those assumptions, the absorptionedge for Si is described by:α_(g)=0, for hv<E _(g), andα_(g) A _(g)(hv−E _(g))², for hv≧E _(g),whereE _(g) =E _(g)(T)=E _(g)(0)−(aT ²)/(T+B)is the band-gap energy of Si as a function of temperature (see Thurmond,1975), T is temperature, hv is the photon energy, and A_(g) is aconstant. Semiconductors are typically never perfectly transparent belowthe band edge due to absorption caused by free carriers. This absorptionis represented by the term:α_(f) =A _(f) T ²,where A_(f) is a constant. The total absorption is given by:α=α_(g)+α_(f).Finally, for the TS measurement configuration, the band-edge spectra aregiven by:TS=((1−R ²)e ^(−αd))/(1−R ² e ^(−2αd)),where R is the reflectivity at the wafer surface and d is the waferthickness.

Band-edge spectra simulations using the above equation for TS are shownin FIG. 8 for a 40 mil silicon wafer. For the purpose of thesesimulations, the band-gap parameters used are E_(g)(0)=1.12 eV,a=0.000473 eV/K, and B=636 K (see Thurmond, 1975) and the otherparameters used are A_(g)=1,000 cm⁻¹eV⁻², A_(f)=0.000004 cm⁻¹K⁻², andR=0.313. These parameters may vary depending on the type of doping andthe doping level of the substrate. In addition, the total absorption or,equivalently, the total transmission depends upon the wafer thickness.Therefore, accuracy may be improved by providing a separate calibrationcurve for each doping type and level and for each wafer thickness.However, given a batch of wafers with uniform thickness and dopinglevels, this measurement technique will have a 1° C. reproducibilitybetween wafers. The simulated spectra shown in FIG. 8 cover thetemperature range from 20° C. to 320° C. The temperature of eachspectrum is listed at the right-hand side of the plot, with the lowesttemperature corresponding to the left-most spectrum. The temperatureincreases 50° C. per spectrum moving to the right where the right-mostspectrum corresponds to 320° C. Those simulated spectra depict, as afunction of wavelength and temperature, the fraction of incidentradiation that passes through the substrate and emerges from theopposite side. The range of wavelengths is shown for which thetransmission changes from essentially no transmission (approximately 0)to maximum transmission (approximately 0.45 to 0.50 depending on thetemperature). When the photon energies are greater than the band-gapenergy, the light is absorbed within the substrate, and when theenergies are below the band-gap energy, the light is transmitted throughthe substrate. The transmitted light is analyzed by the spectrometer,and from a determination of the wavelength at which the onset oftransparency occurs, the substrate temperature may be inferred.

The accuracy of the determination of substrate temperature can beimproved through the use of additional information. Such additionalinformation includes: (1) the extent of process chamber use since itsmost recent cleaning, (2) condition of the wafer surface, (3) wafer type(i.e., p-type or n-type and impurity concentration), (4) thecharacteristics of any surface coatings on the wafer, and (5) the sizeof the measurement elements in comparison to the wafer size and thesizes of any features on the wafer.

A first embodiment of the present invention is shown in FIG. 4. Itcomprises a radiation source with an emission spectrum that includes atleast the range of wavelengths of interest as shown in FIG. 8, and awavelength sensitive detection system utilizing a spectrometercomprising an acousto-optic tunable filter (hereinafter AOTF), and, asshown in FIG. 4, a two-dimensional (hereinafter 2-D) photo-detectorarray (e.g., a 2-D charge-coupled-device (CCD) array or a 2-Dcharge-injection-device (CID) array). As would be appreciated by one ofordinary skill in the art, the band-gap energy can be determined as afunction of temperature from FIG. 8.

A CID array has two distinct advantages relative to a CCD array for thepurposes described herein.

Firstly, a CID array is not subject to “blooming,” which may occur whena pixel is saturated and light intensity “spills” over into adjacentpixels. Secondly, pre-selected pixels within the pixel array may besampled without scanning the entire pixel array. However, CCD arrays aretypically faster and more sensitive than their CID counterparts. CIDarrays may have a maximum pixel interrogation frequency of about 100 kHz(with zero gain). As the interrogation frequency decreases, the gainincreases. For example, a gain of about 50 is attainable for aninterrogation frequency of about 33 kHz. However, CCD arrays may be usedat frequencies as high as about 100 kHz.

One advantage of the present invention is due to the use of an AOTF toreplace mechanically rotated grating/single detector methods. In doingso, superior speed can be achieved over traditional methods whileobtaining a 2-D representation of the temperature distribution across asubstrate.

Furthermore, high speed extraction of temperature information fromband-gap spectra is attainable using a method that utilizes digitalfilters based upon a higher-order derivative of the spectrum. The methodpresented herein can significantly reduce the time necessary fortemperature extraction and minimize interference effects that areinherent to prior temperature extraction methods.

With these improvements, the BET measurement system is capable of highlyresolved spectral measurements with short response times. In fact,response times less than 10 msec are possible. Such response timesrepresent an improvement over past technology by a factor of onehundred. With the advent of this technology, improved spatialtemperature control of semiconductor substrates will be possible.

A first embodiment of the system of the present invention is illustratedin FIG. 4. An optical system 195 views a substrate being processedthrough an infrared (IR) transmitting window 120 b. In the illustratedembodiment the window is located in an upper surface of the processchamber in an ICP or ESRF plasma processor, but other locations arepossible. The ICP and ESRF plasma processors include at least oneinduction coil 129, and an ESRF processor includes an electrostaticshield 128 as well. Ideally the axis of the optical system coincideswith the axis of either the substrate or the wafer chuck.

The optical system 195 of FIG. 4 comprises: (a) a band-pass filter 127that passes (with minimal attenuation) signals at all wavelengthsbetween 0.95 and 1.25 μm and, as nearly as possible, completelyattenuates signals at all wavelengths outside this range; and (b) aneutral density filter or a mechanical iris 200, either of which may beelectrically controlled, that permits uniform adjustment of theintensity of those signals at wavelengths between 0.95 and 1.25 μmtransmitted by the band-pass filter 127; and (c) a lens system 110 bincluding multiple elements and having a field of view encompassing theentire substrate; and (d) a 2-D detection array 145 (including either aCCD array or a CID array) on which the IR-transmitting lens system 110 bforms an image of the wafer 135, by means of the IR radiationtransmitted through the wafer 135 at the multiplicity of measurementsites as illustrated in FIG. 9.

The band-pass filter 127 improves the signal-to-noise ratio (hereinafter“S/N ratio”) of the measurement system by reducing to acceptable levelsthe effect of radiation at wavelengths not between 0.95 and 1.25 μm, therange of interest herein, on the detection array 145. The neutraldensity filter or mechanical iris 200 provides a means by which theintensity of the IR radiation, with wavelengths between 0.95 and 1.25μm, that impinges on the 2-D detection array 145 can be reduced asrequired to assure that no element of the 2-D detection array 145 issaturated due to the IR radiation that impinges upon it. In this way,erroneous data due to the saturation of individual elements of the 2-Ddetection array 145 is prevented.

The measurement system according to the present invention uses the TSmode arrangement generally shown in FIG 1D. A schematic representationof one embodiment of the present invention is shown in FIG. 4. Itincludes a broad spectrum light source 100, an acousto-optical tunablefilter (hereinafter AOTF) 140, the wavelength sensitive optical system195 described above, and a lock-in amplifier 150.

A broad spectrum light source 100 (e.g., a tungsten-halogen light sourceor an array of IR light emitting diodes (hereinafter “LEDs”)) emits IRradiation that is focused by lens 110 a (either a single-ormulti-element lens) onto the entrance aperture of collimator 111. Theradiation passing through collimator 111 is periodically chopped (i.e.,interrupted) by the mechanical chopper 105 driven by the motor 155. Theradiation that passes through the mechanical chopper 105 impinges on theinput aperture of the AOTF 140, which is driven by the radio frequency(hereinafter “RF”) driver 141. The frequency of the signal from the RFdriver 141 determines the narrow band of frequencies that will passthrough the AOTF 140, which has the capability to select signals havingwavelengths within the range from about 0.95 μm to about 1.25 μm with aresponse time of approximately 5 μsec. The angle at which the radiationwith the selected wavelength leaves the AOTF 140 depends, in general, onthe wavelength. However, it is advantageous for all IR radiation thatleaves the AOTF 140 to travel in the same direction when it entersreaction chamber 125 through IR-transmitting vacuum window 120 a. Toachieve this end, the prism 184 is included in the optical path betweenthe AOTF 140 and the IR-transmitting vacuum window 120 a.

In one embodiment, the IR radiation passes through IR-transmittingvacuum window 120 a and impinges upon optical beam splitter 130, whichdivides the IR radiation into plural parts (either equal or dissimilar),the number of parts being determined by the number of measurement siteson the wafer 135 at which the temperature is to be determined. In analternate embodiment, in which only one measurement site is used, theoptical beam splitter 130 is omitted. FIG. 4 shows a division into onlytwo equal parts for simplicity, but a division into many (e.g., ≧10)equal parts is possible. The IR-transmitting vacuum window 120 amaintains the vacuum integrity of the reaction chamber 125. Some of theIR radiation passes through the wafer 135, through the plasma 196, andimpinges on the wavelength sensitive optical system 195.

An additional embodiment of the invention is shown in FIG. 5. In theembodiment of FIG. 5, the AOTF 140 selects the IR radiation to beanalyzed after it has passed through the process chamber 125 (ascontrasted to FIG. 4 discussed herein above). The embodiment of FIG. 5uses the broad spectrum light source 100 described above but does notrequire a band-pass filter 127, because in the location shown in FIG. 5,the AOTF 140 rejects all wavelengths except those at each frequencyselected by the AOTF 140, thereby greatly improving the signal-to-noiseratio.

In an alternate embodiment derived from the embodiment shown in FIG. 5,the radiation emerging from the AOTF is not propagated by the prism 184in the same direction independent of its wavelength. In yet anotheralternate embodiment, the prism 184 is omitted altogether. For suchembodiments, it is possible to unambiguously associate an element orgroups of elements of the 2-D detection array 145 with an individualmeasurement site on the substrate 135 and an individual wavelength asdetermined by the AOTF 140 and the RF driver 141, thus, simplifying thedesign of the interrogator 190.

In FIG. 6, the IR radiation passes through vacuum window 120 andimpinges upon optical beam splitter 130, which divides the IR radiationinto at least two parts (e.g., 10 parts, where the number of parts isdetermined by the number of sites on wafer 135 at which the temperatureis to be determined). Vacuum window 120 maintains the vacuum integrityof reaction chamber 125. Optical fibers 199 a conduct IR radiationthrough wafer chuck 182 to the under side of each of the at least twosites on wafer 135 at which the temperature is to be determined. Some ofthe IR radiation passes through wafer 135, through plasma 196 andthrough an aperture in drive electrode 185 and silicon electrode 183 andthrough replaceable windows 198 to optical fibers 199 b. Replaceablewindows 198 prevent deposits from being formed on the exposed ends ofoptical fibers 199 b.

The IR radiation collected by optical fibers 199 b is conducted by themto optical vacuum feedthroughs 195 a and 195 b, optical filters 173 band 173 d, lenses 110 b and 110 c, and filters 173 a and 173 c, whichfocus the IR radiation onto photodiodes 187 a and 187 b, respectively.In an alternative embodiment, filters 173 a and 173 c are omitted. Inyet another alternate embodiment, filters 173 a and 173 c are used butfilters 173 b and 173 d are omitted. In a further alternate embodiment,filters 173 a, 173 b, 173 c and 173 d are omitted. The amount ofradiation coupled to the photodiodes 187 a and 187 b, thus is controlledto prevent saturation of the detector (or array as described below). Thefilters pass all wavelengths in a desired range (e.g., between 0.95 mmand 1.25 mm) and as nearly as possible completely attenuate allwavelengths outside the desired range. In an embodiment in which thosefilters do not prevent saturation of the detectors, a suitable neutraldensity filter (e.g., an electrically controlled neutral density filter)is included with each band-pass filter. Filters of the types describedherein are well known to persons of ordinary skill in the art. In afurther alternate embodiment, neutral density filters can be replaced bya mechanical iris for limiting the amount of light passing therethrough.

Optical vacuum feedthroughs 195 a and 195 b maintain the vacuumintegrity of reaction chamber 125. In an embodiment in which focussinglenses are included in the packaged photodiodes, separate lenses 110 band 110 c are omitted. The output of each of photodiodes 187 a and 187 bis selected sequentially by interrogator 190 according to a protocolprovided by computer 160 and is conveyed to lock-in amplifier 150. If alock-in amplifier with a sufficient number of input channels is used,interrogator 190 is not necessary. The output signal from lock-inamplifier 150 is sent to computer 160, which stores the data for each ofphotodiodes 187 a and 187 b. After output data for photodiodes 187 a and187 b have been stored in computer 160, computer 160 sends a signal toRF driver 141 for acousto-optical filter 140 and the RF drive frequencyapplied to acousto-optical filter by RF driver 141 is changed to anotherfrequency (e.g., the second of ten pre-selected frequencies). Whencomputer 160 has received data for diodes 187 a and 187 b correspondingto all pre-selected frequencies, it uses a program stored in its memoryto calculate the temperature at the wafer site corresponding tophotodiode 187 a and at the wafer site corresponding to photodiode 187b. Such temperature measurements can be recorded in volatile ornon-volatile storage.

A fourth embodiment of the feedback system of the present invention isshown in FIG. 7. In this embodiment, optical fibers conduct the IRradiation from optical vacuum feedthroughs 195 a and 195 b to lens 110 dthrough filter 173 which focuses the radiation on charge-coupled-device(CCD) array or charge-injection-device (CID) array 145 which may beeither a linear array or a two-dimensional array. The output of eachelement of CCD or CID array 145 is selected sequentially by interrogator190 according to a protocol provided by computer 160 and is conveyed tolock-in amplifier 150. The filter 173 should pass all wavelengthsbetween 0.95 mm and 1.25 mm and as nearly as possible completelyattenuate all wavelengths outside this range. If the filter does notprevent saturation of the CCD or CID array, it may be necessary toinclude with the band-pass filter a suitable neutral density filter. Anelectrically controlled neutral density filter may be used. Filters ofthe types described herein are known to persons of ordinary skill in theart. Except as noted here, the fourth embodiment is the same as thethird embodiment.

All of the embodiments described herein may also be realized using anAOTF having integral fiber optic input and output pigtails. When an AOTFof this type is used, some modifications of the optical elementsproximate to the AOTF 140 shown in FIG. 4 or FIG. 5. For example, in theembodiment shown in FIG. 5, fiber collimators/focusers like the SMA 905or SMA 906 manufactured by OZ Optics, Ltd. might be used advantageouslywith a pigtailed AOTF in conjunction with or in place of collimator 111and prism 184. Such modifications would be understood by a person ofordinary skill in the art and are, of course, consistent with the spiritof this invention.

The measurement procedure begins when the equipment operator enters astart command by means of the input terminal (e.g., keyboard 422 ormouse 424) of the computer 160. RF driver 141 then sends to AOTF 140 asignal that selects the first narrow band of IR wavelengths for passagethrough the wafer 135. The output of each element of the 2-D detectionarray 145 is selected sequentially by the interrogator 190 according toa protocol provided by the computer 160 and is conveyed to the lock-inamplifier 150 and thereafter to the computer 160, which stores the datafor each element of the 2-D detector array 145. (If a lock-in amplifierwith a sufficient number of input channels is used, the interrogator 190is not necessary.) After output data for each element of the 2-Ddetection array 145 have been stored in the computer 160, the computer160 sends a signal to the RF driver 141 for the AOTF 140 and the RFdrive frequency applied to the AOTF 140 by the RF driver 141 is changedto another of, perhaps, ten pre-selected values. The computer 160,having received data for all elements of the 2-D detector array 145corresponding to all of the pre-selected frequencies, calculates thetemperatures at the wafer sites corresponding to the respective elementsof the 2-D detection array 145. After calculating the temperature, thecomputer 160 regulates the temperature distribution. The computer maydirect the wafer chuck heater controller 180 a to adjust the powerdelivered to the multi-element substrate heater 181 a to cause thesubstrate temperature to become either more or less uniform. Thecomputer 160 may also direct the wafer chuck cooler controller 180 b toadjust the multi-element substrate cooler 181 b to cause the substratetemperature to become more or less uniform. To regulate cooling, themultiple element substrate cooler 181 b within the wafer chuck includesplural channels through which the flow of a coolant is controlled by thewafer chuck cooler controller 180 b. In an alternate embodiment, anarray of thermoelectric coolers are embedded in the chuck.

Although the above discussion has assumed a spectral resolution of 30nm, it is possible to obtain a spectral resolution of 3 nm if themeasurement speed (i.e., the interrogation or sampling frequency) isreduced by an order of magnitude. This reduction in measurement speedproduces system response times of 20 msec to 1 sec depending upon thelight source (and the modulation frequency for lock-in detection). Ingeneral, the S/N ratio is greater when using the TS mode rather than theDRS mode (in particular, for silicon wafer temperature measurement).However, in the event that the S/N ratio is low, it can be improved byusing the lock-in amplifier 150. The light source is modulated (e.g.,using a mechanical chopper which communicates with the computer 160) andthe resultant signal is amplified by the lock-in amplifier 150. In thismanner, the signal can be extracted from the noise by observing theresponse occurring at the frequency determined by the chopper 105.However, the speed of the measurement becomes limited by the frequencyof the mechanical chopper 105 (coupled to the broadband light source100) and the subsequent lock-in amplifier 150.

In an alternate embodiment, the broad spectrum light source 100 shown inFIG. 4 includes plural infrared (IR) LEDs, because they are capable ofresponding to significantly higher modulation frequencies. For thatreason, they can greatly improve the measurement speed. Accordingly, anyother IR light source compatible with a high modulation frequency andhaving a broad spectrum output may also be used.

In still another embodiment, light source 100 in FIG. 4 is replaced byan array comprising on the order of ten laser diodes (e.g.,InGa_(x)P_(1−x) laser diodes with different values of the parameter x),each of which emits IR radiation over a very narrow range ofwavelengths. The wavelength emitted by the nth laser diode isapproximately given by λ(n)=0.95+(0.04)n μm where n is an integer with avalue between 0 and approximately 9, but other relationships between theemitted wavelengths are possible. Consequently, the ten laser diodesprovide ten approximately equally separated wavelengths that span therange of wavelengths of interest for this application (approximately0.95 to 1.25 μm); so the AOTF 140, RF driver 141, and prism 184 are notnecessary. However, RF driver 141 is replaced in this embodiment by amulti-output diode controller that sequentially causes one (and onlyone) of the approximately ten laser diodes to emit IR radiation.

Lastly, the high speed measurement (update time <100 msec) of substratetemperature at a multiplicity of pre-arranged spatial locations on thesubstrate enables the chamber thermal characteristics to be optimized atthe substrate. Moreover, with this rapid temporal response, it ispossible to adjust the spatial distribution of the substrate temperatureas the wafer is being processed.

Only a fraction of the light at each wavelength is transmitted throughthe substrate whereupon it is received by the analyzer (e.g., aspectrometer). The AOTF 140 is capable of rapidly tuning the pass-bandwavelength across the pre-selected spectral range (e.g., fromapproximately 0.95 μm to 1.25 μm). For each wavelength in the scansequence the transmission light intensity is recorded using the 2-Ddetection array 145 shown in FIG. 4. The temperature at each measurementsite on the substrate 135 is then obtained from the transmissionspectrum using any of several known techniques to obtain apre-determined calibration curve.

As already described, the system uses a broad band light source 100(e.g., (1) a tungsten-halogen stabilized light source, or (2) an arrayof IR LEDs, or (3) an array of laser diodes). Due to the physical sizeof a conventional lamp filament, the coupling efficiency of the lightinto the AOTF 140 is low. Furthermore, if there are n measurement sites,only 1/n of the light intercepted by the AOTF 140 is coupled to theoptical fibers 199 a for each measurement site. Therefore, the lock-inamplifier 150 is generally required. When using a tungsten-halogenstabilized lamp, the light source is modulated at 1 to 2 kHz using themechanical chopper 105. Lock-in detection is used to remove theincoherent signal (i.e., noise) due to any ambient background light thatmay impinge on the 2-D detection array 145 shown in FIG. 4. An advantageto using the tungsten-halogen stabilized light source is its relativelylow cost, and its ability to provide a continuous spectrum across thespectral range of interest, (e.g., 0.95 μm to 1.25 μm). However, asstated, the tungsten-halogen lamp is less efficient in coupling light tothe optical fibers 190 a than some other sources (e.g., laser diodes).

An important part of a lock-in amplifier is a low-pass filter, which maybe characterized either by its upper half-power frequency (i.e., −3 dBfrequency) or its time constant. The time constant is ½πf_(c), wheref_(c) is the −3 dB or cutoff frequency of the filter. Traditionally, thelow-pass filters of lock-in amplifiers have been characterized by theirtime constant. (The concept of a time-constant is relevant here, becausethe output of the lock-in amplifier will be relativelytime-independent.) The time-constant reflects how slowly the outputresponds to a change in the input, and, consequently, the degree ofsmoothing. A greater time-constant causes the output signal to be lessaffected by spurious causes and, therefore, to be more reliable. Hence,a trade-off must be considered because real changes in the input signaltake many time constants to be reflected at the output. This is becausea single-section RC filter requires about 5 time constants to settle toits final value. It is obvious that faster measurements require shortertime-constants and, therefore higher cutoff frequencies for the filters.Therefore, the conventional chopper at a chopping frequency ofapproximately 1 kHz provides a response time of approximately 5 msec foreach wavelength increment at each measurement site. Hence, such amechanical chopper-based design is a suitable for measurement of thesubstrate temperature every 50 msec for each selected band of IRwavelengths. (This result assumes measurements at ten sites and that thedata extraction algorithm requires approximately 0.01 msec permeasurement.). Therefore, for ten measurement sites and ten wavelengthincrements, approximately 500 msec is required for a complete scan ofthe substrate. An alternate embodiment obtains the data for allmeasurement sites and all wavelength increments according to otherprotocols.

Due to limitations imposed by the mechanical chopper 105, the modulationfrequency is constrained to values much lower than those attainable withLEDs. LEDs, operating in the range of wavelengths between 0.95 μm and1.25 μm, typically have a spectral bandwidth of 10 to 30 nm. Therefore,approximately 9 to 10 LEDs will be necessary to span the wavelengths ofinterest. For improved S/N ratio, lock-in detection can be used whereinLEDs may be modulated at up to 200 MHz (a significant improvement overthe combination of the mechanical chopper 105 and the tungsten-halogenbroad band light source). Hence, the LEDs can solve many of the issuesrelated to speed and light coupling. Although LEDs have considerablyless total optical power than a typical tungsten-halogen light source,their power output in the portion of the spectrum of interest here iscomparable. One additional advantage to using LEDs is that a separateLED per optical port (or channel) may be used to improve the S/N ratio.A disadvantage of using LEDs is their potentially reduced stability andcost. Either way, the background light source spectrum will be recordedas the reference spectrum at each measurement interval and eachmeasurement site.

Use of a modulation frequency of approximately 100 kHz can provide aresponse time for each measurement on the order of 0.05 msec, whichcorresponds to about 0.06 msec per measurement when the data acquisitiontime is included. Hence, it is possible to obtain a total measurementresponse time of approximately 6 msec for ten wavelength increments andten measurement sites.

Method for Extracting Temperature Information

An important part of any technology relating to band-gap thermometry isthe method of extracting temperature from the spectra. The presentinvention utilizes a digital filter that is based upon a higher-orderderivative of the spectrum. Other means of extraction (e.g., a methodbased on wavelet transforms) may be possible. The primary advantage ofthe digital filter is its speed. This is important for achieving the 10to 100 msec update times for fine-grain control. Current band-edgethermometry technology updates temperature on the order of once persecond, in which case the computational speed of current personalcomputers is not an issue. The present invention, however, increases themeasurement frequency by at least a factor of ten, without approachingthe computational limits of those computers.

By providing accurate measurements, the system can use the substrate'stemperature as a feedback in a control loop. Thus, the present inventionprovides a method to control the temperature distribution on a substrateduring processing. In conjunction with the system described in theco-pending patent application entitled “Multi-zone resistance heater,U.S. Ser. No. 60/156,595” the contents of which are incorporated hereinby reference, the multi-site temperature measurement system can providean improved feedback control of substrate temperature. In thatcombination, the control signal is used to adjust the heating and/orcooling of individual zones (or sectors) pre-designed within the chuck(or substrate holder). Due to the speed of the measurement system (<100msec) relative to the thermal response to heating/cooling adjustments(˜1-2 seconds), information obtained from successive temperaturemeasurements (e.g., rate of change or first derivative of thetemperature with respect to time) can provide information to the designof a robust control algorithm.

A computer system 160 shown in FIG. 10 monitors the temperature andsignals any combination of heating and/or cooling zones to increase ordecrease the current heat flux. In particular, the power to each heatingor cooling element and/or the coolant flow rate can be adjusted toprovide for the designated heat flux either into or out of an individualzone. The computer 160 of FIG. 10 implements the method of the presentinvention, wherein the computer housing 402 houses a motherboard 404which contains a CPU 406, memory 408 (e.g., DRAM, ROM, EPROM, EEPROM,SRAM, SDRAM, and Flash RAM), and other optional special purpose logicdevices (e.g., ASICs) or configurable logic devices (e.g., GAL andre-programmable FPGA). The computer 160 also includes plural inputdevices, (e.g., a keyboard 422 and mouse 424), and a display card 410for controlling monitor 420. In addition, the computer system 160further includes a floppy disk drive 414; other removable media devices(e.g., compact disc 419, tape, and removable magneto-optical media (notshown)); and a hard disk 412, or other fixed, high density media drives,connected using an appropriate device bus (e.g., a SCSI bus, an EnhancedIDE bus, or a Ultra DMA bus). Also connected to the same device bus oranother device bus, the computer 160 may additionally include a compactdisc reader 418, a compact disc reader/writer unit (not shown) or acompact disc jukebox (not shown). Although compact disc 419 is shown ina CD caddy, the compact disc 419 can be inserted directly into CD-ROMdrives which do not require caddies. In addition, a printer (not shown)also provides printed listings of the temperature of the substrate, inone or more dimensions over time.

As stated above, the system includes at least one computer readablemedium. Examples of computer readable media are compact discs 419, harddisks 412, floppy disks, tape, magneto-optical disks, PROMs (EPROM,EEPROM, Flash EPROM), DRAM, SRAM, SDRAM, etc. Stored on any one or on acombination of computer readable media, the present invention includessoftware for controlling both the hardware of the computer 160 and forenabling the computer 160 to interact with a human user. Such softwaremay include, but is not limited to, device drivers, operating systemsand user applications, such as development tools. Such computer readablemedia further include the computer program product of the presentinvention for measuring the temperature of a substrate. The computercode devices of the present invention can be any interpreted orexecutable code mechanism, including but not limited to scripts,interpreters, dynamic link libraries, Java classes, and completeexecutable programs.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that, within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

1. An apparatus for measuring temperatures of plural physicallyseparated locations on a substrate in a plasma processing system,comprising: a substrate holder for holding a substrate to be processedin the plasma processing system; a broad band light source on a firstside of the substrate holder for coupling light to the substrate to beprocessed said broad band light source emitting at least a range ofwavelengths for which light transmission through the substrate changesfrom essentially no transmission to essentially a maximum transmission;and plural physically separated light collectors on a second side of thesubstrate holder, opposite the first side, for collecting a portion oflight transmitted through the substrate held by the substrate holder. 2.The apparatus as claimed in claim 1, wherein the light source comprisesa modulated light source.
 3. The apparatus as claimed in the claim 2,wherein the modulated light source comprises an electrically modulatedlight source.
 4. The apparatus as claimed in the claim 2, wherein themodulated light source comprises a mechanically modulated light source.5. The apparatus as claimed in claim 4, wherein the mechanicallymodulated light source comprises: a broadband light source; and amechanical chopper.
 6. The apparatus as claimed in claim 3, wherein theelectrically modulated light source comprises an electrically modulatedbroadband light source.
 7. The apparatus as claimed in claim 3, whereinthe electrically modulated light source comprises plural LEDs.
 8. Theapparatus as claimed in claim 3, wherein the electrically modulatedlight source comprises plural lasers.
 9. The apparatus as claimed inclaim 1, further comprising an acousto-optic tunable filter interposedbetween the light source and the substrate holder.
 10. The apparatus asclaimed in claim 1, further comprising an acousto-optic tunable filterinterposed between the substrate holder and the plural light collectors.11. The apparatus as claimed in claim 1, wherein the plural lightcollectors comprise plural photodiodes.
 12. The apparatus as claimed inclaim 1, wherein the plural light collectors comprise a charge-coupleddevice array.
 13. The apparatus as claimed in claim 1, wherein theplural light collectors comprise a charge-injection device array. 14.The apparatus as claimed in claim 1, further comprising an errordetection circuit for detecting a temperature difference between (1) ameasured temperature of a portion of the substrate corresponding to alocation of one of the plural physically separated light collectors and(2) a target temperature for the portion of the substrate correspondingto the location of the one of the plural physically separated lightcollectors.
 15. The apparatus as claimed in claim 14, wherein thesubstrate holder comprises at least one of a heating element and acooling element for changing a temperature of the portion of thesubstrate corresponding to the location of the one of the pluralphysically separated light collectors based on an output of the errordetection circuit.
 16. The apparatus as claimed in claim 14, wherein thesubstrate holder comprises at least one of a heating element and acooling element for changing a temperature of a portion of the substratenot corresponding to the location of the one of the plural physicallyseparated light collectors based on an output of the error detectioncircuit.
 17. The apparatus as claimed in claim 1, wherein the lightcollection system comprises a central light detector.
 18. The apparatusas claimed in claim 1, wherein the light collection system comprisesplural, physically separated light collectors.
 19. The apparatus asclaimed in claim 1, wherein the plasma processing system is aninductively coupled plasma processing system.
 20. The apparatus asclaimed in claim 1, wherein the plasma processing system is acapacitively coupled plasma processing system.